DNA Gyrase. Recent work on DNA gyrase has focused on the mechanism of the DNA supercoiling reaction. We have shown that the enzyme works by passing a DNA segment through a transient double-strand break in DNA in an asymmetric way. This mechanism is deduced from two kinds of experimental results: 1) DNA gyrase can unlink catenanes of double-stranded DNA and can also untie knotted DNA; 2) the DNA gyrase-induced supercoiling reaction changes the linking number of DNA in steps of two. We have also isolated a fragment of the gyrase B protein that apparently coexists in E. coli with the whole protein. This fragment is about half the size of the gyrase B protein, and can complement the gyrase A protein to produce a DNA-relaxing (but not supercoiling) activity.